Optical scanning devices which are disposed in a laser printer and the like use a polygon mirror rotated by a motor, or an oscillating mirror (resonance mirror) as described in JP-A-5-127109. For example, an oscillating mirror includes a mirror oscillator coupled to a frame portion through a support shaft portion (torsion beam). A movable electrode is disposed on the mirror oscillator, and a stationary electrode is disposed on the frame portion. An optical scanning device using the oscillating mirror comprises a driving circuit which applies a wave-like driving signal (for example, a sinusoidal signal) to the movable electrode or the stationary electrode. Accordingly, the mirror oscillator is caused to oscillate by an electrostatic force which is periodically produced between the movable electrode and the stationary electrode, and a restoring force of the support shaft portion which is elastically deformed by the electrostatic force. When a laser beam is applied from a light source on the oscillating mirror oscillator, the laser beam reflected by the oscillating mirror is periodically scanned over a photosensitive member.
Regarding the oscillating mirror, even if a driving signal of the same level is given from the driving circuit to the electrodes, an oscillation amplitude (oscillation angle range) of the oscillating mirror fluctuates when the ambient temperature or the like is varied. When the oscillation amplitude of the oscillating mirror fluctuates, the scan width of the laser beam on the photosensitive member is changed. Therefore, the optical scanning device described in JP-A-5-127109 includes an optical sensor which receives the laser beam reflected from the oscillating mirror. Then, the oscillation amplitude of the oscillating mirror is adjusted so as to be maintained constant by a feedback control in which a detection time difference of the laser beam by the optical sensor is compared with a predetermined reference time.